1. Field of the Invention
The present invention relates to an image forming optical system and an image pickup apparatus using the same.
2. Description of the Related Art
In recent years, the popularity of an interchangeable-lens camera such as a single-lens reflex camera in which, an electronic image pickup apparatus such as a CCD (charge coupled device) and a CMOS (complementary metal oxide semiconductor) is used, has been increasing. In an interchangeable lens which is used in the interchangeable-lens camera, making a focusing lens group light-weight and simple, and enabling inner focus by the focusing lens group have been sought. This is because, by making the focusing lens group light-weight and enabling the inner focus for reducing drawing out of the interchangeable lens, it is possible to make it silent at the time of focusing. Accordingly, it is possible to reduce a sound of focusing at the time of still photography, and a sound of focusing which is generated by a frequent focusing movement at the time of video photography, in a case such as of a video photography by the interchangeable-lens camera of recent years in which, a highly defined video-photography function surpassing the conventional digital video camera has been installed.
However, when the focusing lens group is merely made light-weight and simple, it is not possible to prevent deterioration of various aberrations caused due to focusing. For instance, when focusing is carried out by one lens, suppressing fluctuation of aberrations, mainly a chromatic aberration, becomes difficult. Moreover, improving Petzval's sum also becomes difficult. For preventing the deterioration of these aberrations, an arrangement of at least one each of a convex lens and a concave lens, a total of two lenses, or one cemented lens, has been necessary in a conventional focusing group, which has been imposing limitation on making the focusing lens group light-weight.
As a compact image forming optical system of which, an overall length is short, image forming optical systems which include four lens groups namely, a negative lens group (a lens group having a negative refractive power), a positive lens group (a lens group having a positive refractive power), a negative lens group, and a positive lens group as disclosed in Japanese Patent Application Laid-open Publication Nos. Hei 10-48524, Hei 10-20194, and 2008-191385, have been proposed. However, in arrangements in Japanese Patent Application Laid-open Publication Nos. Hei 10-48524, Hei 10-20194, and 2008-191385, it has been extremely difficult to achieve both of making the focusing lens group light-weight and simple, and preventing deterioration of various aberrations caused due to focusing.